1. Field of the Invention
The present invention concerns a display panel in which an indication mark is illuminated by light emission from the rear face and, more in particular, it relates to a display panel suitable to use as a number plate for an indication instrument.
2. Description of the Prior Art
For instance, in an automobile indicator, indication marks such as scales or numerical figures printed on the front surface of a number plate can be seen clearly during night or in a dark circumstance. Specifically, a light permeable substrate is adopted for the number plate and it is formed such that an indication mark is in the form of negative indication (indication mark appears bright) or positive (opposite to the negative) indication and an illumination member comprising lamps is disposed at the back of the number plate (for example, refer to U.S. Pat. Nos. 4,878,387 and 5,047,761).
In the above-mentioned indicator, the brightness of the indication mark becomes not uniform depending on the difference in the distance from the illumination member and it is difficult to see. Further, in view of the heat generation from illumination member, an expensive heat resistant material has to be used for a casing containing the indicator. Further, since a gap has to be large between the illumination member and the casing in view of the heat generation from the illumination member, the size of the casing is enlarged.
As a countermeasure for overcoming such problems, an electroluminescent lamp panel (hereinafter referred to as "EL") is sometimes disposed at the back of the number plate (for example, refer to U.S. Pat. No. 5,036,249 and Japanese Utility Model Laid-Open Sho 62-40518 and Sho 62-124510).
The foregoing problems can be solved by adopting EL for the illumination of the number plate. However, when EL is at the back of the number plate, the position between EL and the number plate has to be aligned accurately for satisfactory illumination of the indication mark on the number plate by the EL emission. In addition, since EL is thin and easy to flex distort, EL has to be protected from the surrounding atmosphere such as external vibrations and a rigid rear frame has been attached at the back of EL for instance. Accordingly, assembling operation between the number plate and EL or the number plate, EL and the rear frame becomes to complicated to make the production troublesome.
An object of the present invention is to overcome the foregoing problem and provide a display panel integrated with EL and, particularly, a display panel suitable to use as a number plate for use in an indicator.